High-strength hot rolled steel plate and manufacturing method thereof

ABSTRACT

The present invention provides a new high-strength and Si—Cr containing hot rolled steel plate provided with higher strength as well as excellent workability and a method for manufacturing the steel plate. The high-strength steel plate can be obtained by controlling the particle size of prior austenite to be 10 μm or less, and properly selecting the coiling temperature. The steel plate obtained includes a retained austenite phase in a volume fraction of 5% to 20%; a martensite phase in a volume fraction equal to or less than 10%; and a bainite phase in the remaining volume fraction. The particle size of the retained austenite particle is 1 μm or less and the retained austenite particles are dispersed uniformly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon the prior Japanese Patent Application No.2007-108759 filed on Apr. 17, 2007, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength hot rolled steel platehaving high tension strength and superior workability, and also relatesto a manufacturing method thereof.

2. Description of the Related Art

Recent demands for a high-strength steel plate which can exhibitsuperior workability will be described, with respect to cars, by way ofexample. In view of environmental protection for the earth, it should berequired to reduce the amount of exhaust gas, such as CO₂ or the like,in the field of automobiles. To this end, it is quite essential tofurther reduce the weight of each car body. In order to achieve theweight reduction of the car body, it is necessary to enhance thestrength of steel plate used for the car body so as to reduce thethickness of the steel plate. In addition, safety for the users shouldbe secured in the car. Also for this purpose, the strength of the steelplate must be further improved.

However, increase of the strength of the steel plate may tend to degradeits workability, and it will be difficult to apply a higher strengthsteel plate to cold working, such as general press molding or the like.

Hot pressing is a hot press working process and usually generates aquite small amount of spring back, thus exhibiting preferable shapefreezing properties. In addition, due to a hardening effect upon the hotpressing, this method can present parts having significantly higherstrength, with high accuracy. However, this process requires heating thesteel plate prior to subjecting it to the hot press working, and alsorequires reduction of the manufacturing scale after the hot pressworking. Thus, this process has possibility to significantly deterioratethe working efficiency. Furthermore, shorter life of the mold, whichshould contact with a heated steel plate, inevitably increase themanufacturing cost.

The elongation of the steel plate after the hot press working will bedecreased, as such a hot pressed member may tend to be broken only dueto slight deformation caused by being subjected to some external force,impact or the like. Therefore, the steel plate of this type is generallyassessed to be of poor impact absorbing ability. Accordingly, it isquite difficult to use such hot pressed parts as key components forsecuring the safety for cars or the like.

As a method for enhancing the strength, reinforcement by a solidsolution treatment, reinforcement utilizing precipitation, reinforcementby grain refinement, and reinforcement utilizing a low-temperaturetransforming phase can be mentioned. It is not possible to manufacturethe steel plate, for which significantly enhanced strength is required,only by employing a reinforcing mechanism, including the solutiontreatment or precipitation requiring addition of a greater amount ofalloys. Even in the case of utilizing the reinforcement by grainrefinement, the improvement of the strength is limited although achievedto some extent. While the reinforcement by utilizing a low-temperaturetransforming phase is highly effective for manufacturing the steel plateexceeding 1200 MPa, this method can not be expected, in order to enhanceductility which can be balanced with such improvement of the strength.

Generally, higher strength of the steel plate may tend to lowerductility, as such degrading the workability.

As known materials having enhanced ductility among high-strength steelplates, there are a dual phase steel plate consisting of ferrite andmartensite phases, and a transformation induced plasticity (TRIP) steelplate consisting of ferrite, bainite and retained austenite phases.

The dual phase steel plate is formed by finely dispersing the hardmartensite phase in the ferrite phase. Due to the highly hard martensitephase, significant work hardening is caused upon transformation, thusproviding higher ductility to the steel plate.

Examples of the TRIP steel plate are described in Patent Documents 1 and2. The steel plate of this type containing the retained austenite phaseexhibits highly excellent ductility and moldability both attributed toworking induced transformation, depending on the amount the retainedaustenite phase and the stability to the deformation.

However, if attempting to obtain the steel plate having strength greaterthan 1200 MPa, delayed fracture may be caused. The term “delayedfracture” means a phenomenon wherein while cracking and/or fracture isnot generated upon working and assembly for respective members, itappears suddenly during use of them. A high-strength steel platedisclosed in Patent Document 3 is intended to provide more preferableanti-delayed fracture properties, by reducing a soft phase, such as theferrite phase, as much as possible, and by controlling the volumefraction of the retained austenite phase to be less than 4%, relative tothe low-temperature transforming phase, such as the bainite phase and/orthe tempered martensite phase.

Patent Document 1: JP-A-No. 60-43425

Patent Document 2: JP-A-No. 9-104947

Patent Document 3: JP-B-No. 3247908

As the steel plate whose elongation properties in the cold working areenhanced while keeping higher strength, the dual phase steel plate andTRIP steel plate as described above can be mentioned.

In the dual phase steel plate, a higher strength can be achieved even inthe case of a smaller additional amount of alloys, as well as moreuniform elongation properties can be obtained due to the work hardening.

The TRIP steel plate exhibits higher ductility and has more excellentdeep drawing properties. Therefore, this material is suitable forproviding a part or member for which a complicated shape, higherworkability and more enhanced strength are required.

The TRIP steel plate described in the Patent Document 1 is manufacturedby a method comprising: creating the ferrite phase in the austenitephase by holding a raw material at 450 to 650° C. for 4 to 20 seconds ina cooling step after rolling, cooling it to a temperature lower than350° C., and coiling it around a rod material.

In the Patent Document 2, in order to promote formation of the ferritephase in the austenite phase in a cooling process after the rolling, araw material is gently cooled at Ar3 to Ar1 or subjected to a rollingcompletion temperature of approximately Ar3 then cooled to a temperaturewithin a range of 350 to 500° C., and is wound around a rod material.

Such a TRIP steel plate has a structure in which the martensite phase,retained austenite phase and/or bainite phase is dispersed in theferrite parent phase, and exhibits excellent strength and elongationproperties.

However, under the condition of C≦0.20% which can ensure the spotwelding properties, only the tensile strength as high as 800 MPa can beobtained, as such more enhanced workability should be desired.Accordingly, it is difficult to manufacture the steel plate havingsignificantly higher strength, under such conditions.

Even in a method of gradually cooling the raw material to a temperaturelower than 500° C. without providing the gentle cooling process en routeafter the rolling, the promotion of creating a fine ferrite phase can beachieved, if setting the rolling completion temperature at a point ofapproximately Ar3. With respect to the material quality of a hot rolledsteel plate which has been subjected to rolling at a temperature ofapproximately A3, however, anisotropy of the material may tend to beundesirably greater.

Moreover, the hot rolled steel plate described in the Patent Document 1exhibits lower rolling workability and has a metallic structure in whichcoarse ferrite particles and retained austenite particles are presentcontiguously because of the temporary stopping for the cooling at apoint of approximately A1.

Hydrogen dissolved in the steel plate, which is likely to be a cause ofthe delayed fracture, is a factor of determining the crystal phase, andis trapped preferentially in the retained austenite phase. Especially,the interface between the martensite phase and ferrite phase having beensubjected to the impact or working, i.e., the working inducedtransformation site, is considered to be a highly possible trapping sitefor hydrogen.

Coarser retained austenite particles will provide a more reduced ratioof the area of the interface between the martensite phase and ferritephase having been experienced the working induced transformation, ascompared with the volume of the retained austenite particles.Consequently, the concentration of hydrogen to be trapped is increased,as such presenting a greater risk of the delayed fracture. If themartensite phase and the retained austenite phase coexist contiguously(in an M-A state), propagation of the fracture is likely to be promoted,thus providing a further increased risk of the fracture.

The high-strength steel plate described in the Patent Document 3 isintended to enhance the anti-delayed fracture properties by limiting theamount of the retained austenite. However, in order to obtain excellentworkability while keeping higher strength, utilization of the retainedaustenite is substantially effective. Accordingly, it is desirable ifthe presence of the retained austenite will not detrimentally affect theanti-delayed fracture properties without providing any limitation asdescribed above.

SUMMARY OF THE INVENTION

To address this challenge, the present inventors have developed a newlow-alloy and higher-strength steel plate and a method of manufacturingthereof, the steel plate having a bainite phase in which seven or moreof the retained austenite particles having a particle size of 1 μm orless are finely dispersed per 10 μm² (the volume fraction is within therange of from 5% to 20%), thereby exhibiting higher strength as well asmore preferred workability and secure anti-delayed fracture properties.

Through many experiments, we have found that a preferably high-strengthsteel plate can be obtained by employing appropriate rolling conditionsand selecting a proper composition of components for the steel plate.Namely, higher strength and excellent ductility as well as secureanti-delayed fracture properties can be provided to a low-alloy steelplate, by subjecting a slab having a proper composition of components torough hot rolling under high pressure conditions, completing rear-stagehigher strain rolling in a finish rolling process under high temperatureconditions, starting a cooling process after air-cooling for severalseconds, and coiling the processed material at an appropriatetemperature.

A high-strength hot rolled steel plate of the present inventioncomprises: a retained austenite phase in a volume fraction of 5% to 20%;a martensite phase in a volume fraction of 0% to 10%; and a bainitephase in the remaining volume fraction, wherein particles constitutingthe retained austenite phase have a particle size of 1 μm or less. Morepreferably, the particle size of prior austenite is 10 μm or less, andthe average aspect ratio of the particles is 2.0 or less.

With the control of the particle size of the austenite crystal after hotrolling to be 10 μm or less (FIG. 3), the lath structure of the bainitephase can be made fine. In addition, with completion of uniform bainitetransformation, the retained austenite particles having a particle sizeof 1 μm or less can be finely and effectively dispersed in the phasewith the density of seven or more particles per 10 μm² (FIG. 8).

In this manner, superior anti-delayed fracture properties can beobtained, even in the case of steel provided with a higher ductility byutilizing working induced plasticity to be caused by a relatively greatamount of retained austenite.

With the control of the aspect ratio of the prior austenite particles tobe 2.0 or less (FIG. 3), anisotropy of the material, which is drawn inboth of the rolling direction and the direction vertical to the rollingdirection, can be reduced, as such enhancing the workability (FIG. 4).

Preferably, the high-strength hot rolled steel plate of the presentinvention has a composition comprising: C (0.13 to 0.21 (% by weight)),Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo(0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), andthe remaining components including Fe and other inevitable impurities.

Such a chemical composition comprising proper types and amounts of theselected components can facilitate formation of the high-strength steelplate which can include the phases described above and exhibit desiredmechanical properties.

Since the alloy elements described above can constitute the desiredsteel plate structure of the present invention in the steps of coolingafter hot rolling, and coiling the cooled material, Cr and Si havinggreater influence on the bainite transformation are included as majorelements. With controlling of amounts of these elements, the bainitetransformation can be promoted, and formation of the martensite phasecan be suppressed, thereby to control the entire phase to have an aimedstrength.

The effect of each component will be described below.

It is preferred that the high-strength hot rolled steel plate has thestructure as described above, and also a plate thickness of 1.0 to 3.0mm, tensile strength (TS) of 1200 MPa or greater, and elongation of 13%or greater (JIS No. 5 test piece).

Namely, this steel plate can possess the structure described above, andhence exhibit greater strength and more excellent elongation properties.

A method for manufacturing a high-strength hot rolled steel plateaccording to the present invention comprises the steps of:

-   (1) preparing a slab (rolling material) having a composition    containing: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2    to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0    to 0.010), S (0 to 0.003), N (0.005 to 0.015), and the remaining    components including Fe and other inevitable impurities;-   (2) roughly rolling a steel material, under the conditions of:    1250° C. or higher of an extraction temperature of a reheating    furnace; 1030° C. or higher of a discharging-side temperature of    roughly rolling mills; and 30% or higher of a reduction ratio for    each of roughly rolling final three passes;-   (3) finish rolling of the steel material under the conditions of:    950° C. or higher of a discharging-side temperature of finish    rolling mills; 40% or higher of a reduction ratio for each of finish    front-stage first to third rolling mills (this is the case of using    six rolling mills, but first to fourth rolling mills are used in the    case of using seven rolling mills) and 0.5 or greater of accumulated    strain in the pressed state due to three rolling mills on a finish    rear-stage; and-   (4) cooling the steel material by air-cooling for 2 to 6 seconds,    followed by water-cooling, and coiling the steel material at a    coiling temperature of 550 to 650° C.

With the purpose of enhancing strength by obtaining a low-alloy baitnitephase due to employment of a temperature history (FIG. 1) formaintaining the temperature, in the steps of rapid cooling after hotrolling by utilizing hot strip milling and coiling the material atpredetermined temperature conditions, a uniform phase of baitnite, inwhich martensite and retained austenite are finely dispersed, can beobtained, by adding Cr and Si as major alloy elements and selecting acomposition containing lower Mn and Ni (FIG. 7( b)).

With control of precipitation of carbides due to addition of Si and withformation of a more uniform baitnite phase, the austenite having carbondensity of 0.8% or higher can be retained in a greater amount. In thisway, a steel plate having enhanced strength and more excellentworkability can be obtained (FIG. 11).

By controlling the hot rolling finish temperature to be 950° C. orhigher, the aspect ratio of the prior austenite particles can becontrolled at 2.0 or less (FIG. 3).

In order to prevent biting failure of a topmost portion of the rollingmaterial into a roll, it is preferred that, upon the finish rolling, areduction amount of a topmost portion of the steel material is reduced,as needed, as compared with an expected reduction amount (or reductionamount originally set for a predetermined rolling), in first to fifthrolling mills (in the case of using six stages of finish rolling mills,while first to sixth rolling mills are used in the case of using sevenstages of finish rolling mills), wherein the reduction amount isincreased by 10% or less, as compared with the expected amount of eachrolling mill. It is also preferred that a length to be rolled in theincreased reduction amount is within 5 m as measured from a bitingposition of the topmost portion of the rolling material.

In order to prevent slip occurrence between the rolling material and theroll during the rolling process, it is also preferred that a specialhigh-grip roll is used as a working roll for each of finish first tothird rolling mills including the final rolling mill.

Our test on the manufacture, which will be described below, demonstratesthat the aforementioned high-strength steel plate can be obtainedreadily by employing the conditioned as provided above.

In the high-strength steel plate of the present invention, the retainedaustenite is incorporated in the baitnite phase in a volume fraction of5% to 20% such that it is finely dispersed with the density of seven ormore particles per 10 μm². Therefore, both strength and workability,which are contrary to each other, can be provided to the steel plate,and excellent anti-delayed fracture properties can also be providedthereto.

According to a method of the present invention for manufacturing ahigh-strength steel plate, the high-strength steel plate described abovecan be readily and securely manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description takenin connection with the accompanying drawings, in which:

FIG. 1 is a graph for schematically showing a temperature history of hotrolling in a manufacturing process of one embodiment of the presentinvention;

FIG. 2 is a photograph of prior austenite particles of a crop at afinish entrance;

FIG. 3 is a photograph of prior austenite particles;

FIG. 4 is a graph showing a relationship of finish rolling temperaturesand anisotropy of elongation;

FIG. 5 is a graph showing a relationship of a rolling schedule and arolling temperature;

FIG. 6 is a graph showing a relationship of a dislocation density and aparticle size of prior austenite particles;

FIG. 7 is a photograph of typical phases of sections;

FIG. 8 is a photograph of phases of sections each obtained by an EBSPmethod for a steel plate manufactured under compositional and rollingconditions according to the present invention, and bright or light colorportions designate retained austenite;

FIG. 9 is a graph showing a deviation of a plate thickness at a distalend of a rolling material;

FIG. 10 is a graph showing a relationship of a coefficient of frictionand a reduction ratio, depending on types of rolls; and

FIG. 11 is a graph showing a balance between the strength and theductility, and a relationship of the ductility and the amount ofretained austenite.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, one embodiment of a sheet steel used to produce parts byworking the same, for which excellent workability and anti-delayedfracture properties are required while keeping tensile strength of 1200MPa or higher, and a manufacturing method of the sheet steel will bedescribed.

The steel plate has a composition containing the following components: C(0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to0.003), N (0.005 to 0.015), and the remaining components include Fe andother inevitable impurities.

As used herein, the term “sheet plate” means a steel plate having athickness of from 1.0 mm to 3.0 mm. The steel plate to be manufacturedunder the above compositional conditions can be mainly used as parts forcars, consumer electrical appliances, electronic equipment and the like,which require higher workability and strength. In addition, the steelplate can also be applied to materials for steel pipes.

First, components of the steel plate will be described.

The amount of carbon (C) should be within the range of 0.13 to 0.21%.

C is the most important component for stabilizing the retainedaustenite. If the amount of C is less than 0.13%, sufficient stabilitycan not be obtained, thus an amount of C of 0.13% or greater should berequired. However, if it exceeds 0.21%, a welded portion becomes toohard and is likely to be broken. Such a situation provides somelimitation of use to the sheet steel to be formed. Therefore, the upperlimit described above is provided to the amount of C. Namely, by settingthe amount of C within the range of 0.13 to 0.21%, it has been foundthat a composite structure which accords with an intention of thepresent invention can be obtained.

The amount of silicon (Si) should be within the range of 0.5 to 2.0%. Sialso serves to stabilize the retained austenite. In addition, Sienhances the strength to be obtained by reinforcement due to solidsolution. If the amount of Si is 0.5% or greater, a preferred compositestructure and material quality can be obtained. A greater amount of Sican increase more retained austenite as well as enhance the stability.However, if the amount of Si exceeds 2.0%, properties for balancing thestrength and the ductility will be saturated, thus the upper limit ofthe Si amount should be set at 2.0% in view of reduction of the cost.

The amount of chromium (Cr) should be within the range of 1.0 to 4.0%.Cr can create the bainite phase, and enhance the strength of the steelplate to be formed therewith.

If the amount of Cr is less than 1.0%, the amount of ferrite is unduelyincreased, as such making it difficult to obtain a steel plate with adesirably higher strength. Therefore, the Cr amount should be 1.0% orgreater. However, if it exceeds 4.0%, the martensite phase is likely tobe produced, thus making the steel plate strength too high and hencerendering the anti-delayed fracture properties insufficient. Therefore,4.0% is set as the upper limit.

The amount of manganese (Mn) should be within the range of 0.2 to 1.0%.If the Mn amount is less than 0.2%, the manufacture of the steel platewill be difficult. Therefore, it should be 0.2% or greater.

In order to attain higher strength, it is preferred to add Mn as much aspossible. However, if it is excessively added, the martensite phase maytend to be produced, thus making it impossible to obtain the intendedstructure according to this invention. Therefore, the upper limit of theMn amount should be set at 1.0%.

The amount of nickel (Ni) should be within the range of 0.02 to 1.0%. Nican enhance the strength of the steel plate by reinforcement due tosolid solution. However, if the amount of Ni is too increased, themartensite phase is likely to be produced. Moreover, inadvertentaddition would lead to increase of the production cost. Thus, the upperlimit should be set at 1.0%.

Molybdenum (Mo) can create the bainaite phase as is similar to Cr, andenhance the strength of the steel plate to be formed therewith. Inaddition, a hydrogen trapping effect due to Mo carbides is useful forproviding anti-delayed fracture properties to the steel plate. However,inadvertent addition would cause unduely restrained recrystallization aswell as lead to increase of the cost. Therefore, the Mo amount should beset within the range of from 0.05 to 0.40%.

In order to enhance the weldability, it is necessary to possibly reducethe amount of phosphorus (P). Thus, the upper limit of this elementshould be set at 0.010%.

Also, in order to enhance the weldability, it is necessary to possiblyreduce the amount of sulfur (S). Therefore, the upper limit of thiselement should be set at 0.003%.

The amount of nitrogen (N) should be within the range of 0.005 to0.015%. As is similar to carbon, nitrogen is useful to stabilize theaustenite phase. However, its excessive existence will cause degradationof the weldability. Thus, the range of this amount should be set at avalue of from 0.005 to 0.015%.

A slab produced to have the composition as described above is thensubjected to hot rolling after heated again or subjected to hot rollingimmediately after casting.

FIG. 1 is a graph for schematically showing a temperature history of hotrolling in a manufacturing process of one embodiment of the presentinvention, in which particles sizes of prior austenite are alsodesignated. The horizontal axis denotes the elapsed time and thevertical axis denotes the temperature.

Upon providing the hot rolling, the extraction temperature of thereheating furnace was set at 1250° C. This temperature was selected topreferentially secure the surface temperature of 950° C. after thefinish, even though some inevitable growth of austenite particles wouldbe caused in the reheating furnace due to such a high temperaturecondition. However, the size or diameter of the austenite particles willbe lessened in the following rolling process. Therefore, it is necessaryto reduce the particle size of the prior austenite as finely as possiblebefore subjecting it to a finishing rolling mill. Accordingly, in aroughly rolling process, the crystal particle size is reduced in advanceto 35 μm or less, by setting the reduction ratio of each of final threepasses for roughly rolling at 30% or greater, at a discharging-sidetemperature of 1030° C. or higher on the discharging side of the roughlyrolling mills. FIG. 2 shows a particle size of prior austenite aftersubjected to the roughly rolling, wherein the processed material was cutby a pre-finish crop shearing machine.

For first to third rolling mills on a finish front-stage (in the case ofusing six finish rolling mills, but first to fourth rolling mills areused in the case of using seven finish rolling mills), the reductionratio per mill is set at 40% or higher. Accumulated strain in thepressed state for three rolling mills of a finish rear-stage is set at0.5 or greater, and the finishing rolling mill discharging-sidetemperature is securely set at 950° C. or higher, so as to render theaustenite particle size equal to or less than 10 μm. In addition,air-cooling is provided for 2 to 6 seconds after the finish rolling,followed by water-cooling. A coiling temperature is set at 550° C. to650° C. In the above air-cooling step, the size of the austeniteparticles is also controlled. Namely, during the hot rolling step, theparticle size of the prior austenite is controlled to be 10 μm or lessbefore post-hot-rolling hot run cooling is started, so as to control thesize of the prior austenite particles to eliminate working strain.

FIG. 3 shows a result of observation for the prior austenite particlesof the steel plate according to the present invention by using an SEMphase observation. An average particle size of the prior austeniteparticles is 9.3 μm, presenting a uniformly granulated structure. Anaverage aspect ratio of the major axis/the short axis is 1.7.

In the case of using low-temperature rolling, such that a rollingcompletion temperature is 950° C. or lower, and employing theaccumulated strain, set at 0.5 or less, of three rolling mills on afinish rear-stage, the austenite particles become larger (10 μm orlarger), and the shape of each austenite particle may tend to be flatdue to rolling, thus causing increased anisotropy. FIG. 4 shows arelationship between the finish rolling mill discharging-sidetemperature (FDT) and anisotropy of elongation. As is seen from FIG. 4,the anisotropy of elongation appears when the FDT is 950° C. or lower.This anisotropy is defined by an equation of IC−LI/(C+L)/2 (L is anelongation in the rolling direction, and C is an elongation in thedirection vertical to the rolling direction). A smaller value to beobtained from this equation shows less anisotropy.

As used herein, the “strain” means a value designated by ε in thefollowing equation:

ε=(h ₀ −h ₁)/{(h ₀ +h ₁)/2}

wherein a difference, between a thickness h₀ of the steel plate on theinlet side and its thickness h₁ on the discharging side, for each stand(each stage, or each pass upon rough rolling), is divided by an averagethickness of the both thicknesses.

As used herein, the “accumulated strain” means a value expressed byε_(c) in the following equation:

ε_(c)=ε_(n)+ε_(n-1)/2+ε_(n-2)/4

wherein strain of each stage (each pass) of the rear-finish three standsis calculated by using a weighted estimation, considering the strengthof each effect to be imposed on the metal phase, and wherein the strainto be generated on a final stage (final pass), front-stage (pre-pass),and pre-front-stage (pre-(front-pass)) is each expressed by ε_(n),ε_(n-1), and ε_(n-2).

In order to carry out high-temperature finish rolling, a temperaturerising process for the steel plate by utilizing heat generated byworking due to the rolling is employed. To this end, it is important toprovide a schedule of high-strain rolling for each rear-stage rollingmill as well as to set the reduction ratio of the front-stage stand at40% or higher. As shown in FIG. 5, it can be seen that the surfacetemperature after finishing will vary, by 80° C., depending on the typeof steel, in the case of using the same rolling size with respect to thereduction ratio while there is a difference in exit thicknesses atroughing mill.

Hot rolling is completed at a temperature of 950° C. or higher, and thematerial is then subjected to air-cooling for 2 to 6 seconds withoutundergoing the post-hot-rolling hot run cooling, so as to reduce thedislocation density in the crystal particles. In FIG. 6, changes in theaustenite particle size and changes in the dislocation density areillustrated, wherein these data are obtained by calculation over aperiod from a finishing F1 rolling mill to starting the hot run cooling,in the case of changing the rolling temperatures for the same type ofsteel. From the drawing, it can be seen that the dislocation density issignificantly influenced by the rolling temperature. It can also be seenthat under high-pressure rolling conditions, the austenite particle sizewill be smaller under lower temperature conditions, provided that theprocessing temperature is equal to or higher than that required for Ar3transformation. However, under such lower temperature conditions, thedislocation density will be higher, thus providing a material withfurther increased anisotropy. Additionally, it can be seen that whilethe dislocation density is significantly reduced due to the hot runair-cooling after rolling, effective results can be obtained byemploying the cooling time within six seconds. In this simulation model,setting the aspect ratio at 2.0 or less can be translated intocontrolling the dislocation density to be at least 2.50E +10 (ρ/cm²) orless (this was confirmed from the results of comparison between actualdata and the simulation model). However, the reduction of thedislocation density leads to increase of the size of the prior austeniteparticle. In order to further reduce the above-described numerical valueof the dislocation density while controlling the prior austeniteparticle size at 10 μm or less, the aforementioned rolling conditions(rolling temperature: 950° C. or higher, and cooling time: 2 to 6seconds) are required.

The simulation model described above is based on Yanagimoto, Morimoto,et al., “Iron and Steel”, vol. 88 (2002), No. 11, and each coefficientof the numerical formulae was reviewed for this application.

While the coiling temperature was set at a value within the range offrom 550° C. to 650° C., the temperature range lower than 550° C. maytend to increase the martensite phase, thus increasing the possibilityof delayed fracture. On the other hand, the temperature rang higher than650° C. will generate more ferrite and pearlite, as such making itdifficult to obtain higher strength. In FIG. 7, sectional phases ofthree-types of high-strength steel plates are shown.

Either phase shown in FIG. 7 is bainite based, in which a photographcorresponding to FIG. 7( a) shows a martensite-rich structure, FIG. 7(b) shows a lower-martensite and relatively fine structure, and FIG. 7(c) shows a ferrite-containing structure. FIG. 7( b) shows a structureobtained according to the present invention.

In the bainite-based structure, austenite is retained in each interfacebetween the prior austenite particles as well as in each packet boundaryand each block boundary, i.e., in the prior austenite particlesthemselves. The retained austenite can be closely and uniformlydispersed into a parent phase such that seven or more of the retainedaustenite particles having a very fine particle size, such as 1 μm orless, are dispersed per 10 μm², by employing the bainite phase as theparent phase and setting the size of prior austenite particles beforetransformation at 10 μm or less. FIG. 8 is a photograph of structures ofsections each obtained by an EBSP method, for the steel plate accordingto the present invention, in which the bainite phase having abody-centered cubic phase and the austenite phase having a face-centeredcubic phase are discriminated by colors. The retained austenite phaseshown by a bright color constitutes a structure in which seven or moreof the retained austenite particles having a particle size of 1 μm orless are finely and uniformly dispersed per 10 μm².

Due to such control for the hot rolling, the bainite phase can beobtained, in which the retained austenite particles are finely anduniformly dispersed.

If rolling a high-strength thin plate material (having a thickness of 2mm or less) under high-strain and high-reduction-ratio conditions, abiting failure at a plate top portion and/or a slip between a roll and arolled material during the rolling operation is likely to occur. It hasbeen found that from the rolling results, binding properties at atopmost portion of the rolling material is not problematic in the caseof using a material of TS less than 1000 MPa at the reduction ratio of40 to 50% per each rolling mill. On the other hand, the binding failureat the topmost portion of the rolling material will be likely tofrequently occur (rate of occurrence: 50%), at the final rolling milland the first to second rolling mills of the front-stage rolling mills,if using a material of TS greater than 1000 MPa. As a measure foraddressing this problem, we have attempted to elevate the roll grindingfinish roughness Ra up to 1 μm (ordinarily 0.5 μm) in order to raise thecoefficient of roll friction, so as to obtain the coefficient offriction (μ) during rolling of 0.4 (ordinarily 0.3). In addition, wehave reduced the flow amount of the roll cooling water in order not tounduely decrease the temperature at the topmost portion of the rollingmaterial. However, securely effective results could not be obtained.Accordingly, as shown in FIG. 9, we have attempted to render the topmostportion of the rolling material thinner, over a place within the rangeof 5 m from the discharging side of the rolling mill, so as to make athinner plate thickness (by 10% of a thickness finally expected).Thereafter, an inclination up to the expected plate thickness wasprovided to the plate material.

As a result, the binding failure was drastically decreased (rate ofoccurrence: 0%). In addition, the setting reduction amount employed in arange from the finish front-stage rolling mills to the rolling millslocated before the finish final rolling mill was set at a value to beobtained by adding 10% or less of an expected set value thereto. Thereduction setting time is set within two seconds from a biting site ofthe topmost portion of the plate material into the rolling mill.

With respect to the slip between the rolling mill and the rolledmaterial during a rolling process, if rolling a material of TS greaterthan 1000 MPa, under high temperature and high pressure conditions, withthe final plate thickness being set less than 2 mm, slip is likely tooccur at the final rolling mill and the rolling mill located just beforethe final rolling mill. As a phenomenon of this situation, a metallicsound is generated during the rolling process, the rolling load of therolling mill upon occurrence of the slip is drastically decreased to 50%or so. Then, the rolls become idling, and the rolled plate can not beadvanced. At this time, when pulling out a roll from each rolling milland measuring the rolling roughness Ra of the roll, it is measured to beless than 0.1 μm, showing a state wherein the rolled material and theroll are likely to slip on each other. To address such a situation, wehave employed special high-grip rolls. As a result, the occurrence ofthe slip could be avoided completely. The rolls are each formed byuniformly dispersing micro-carbide particles (particle size: less than 1μm) over the whole surface of the roll. These carbide particles can beutilized as spikes and supported by a hard base material. In addition,even through the micro-carbide particles will be worn away from thesurface, micro-oxide particles will successively appear from below, thusmaintaining a stable coefficient of friction, thereby to prevent theoccurrence of slip. As shown in FIG. 10, changes of the coefficient offriction due to the rolling process is maintained in a more suitablerange (approximately 0.3) as compared with commonly known rolls.

FIG. 11 is a graph showing a relationship between the volume fraction(Vγ) of the retained austenite in the heat rolled steel plate producedby the manufacturing process shown in FIG. 1 and data obtained by thetensile test. FIG. 11( a) shows a relationship between the volumefraction Vγ and (the tensile strength×elongation). FIG. 11( b) shows arelationship between the volume fraction Vγ and the elongation. As isseen from the drawing, in the range of from 5 to 20% of the volumefraction of the retained austenite, as the volume fraction Vγ isincreased, the data of the tensile strength×elongation and theelongation alone are improved. The metallic phase corresponding to thedata can be considered as the lower-martensite fine bainite phase asshown in FIG. 7( b).

The present invention was made on the above empirical basis.

EXAMPLES

Hereinafter, examples of the present invention will be described.

Slab materials (rolling materials) were prepared from melted steelhaving each chemical composition shown in Table 1 by using a forgingmethod or continuous casting method. Subsequently, these slab materialswere heated again, and subjected to hot rolling, so as to obtain hotrolled steel plates, respectively. Table 2 shows respective conditionsof the hot rolling and properties of the materials.

TABLE 1 Chemical composition (%) N Classes Types C Si Mn P S Cu Ni Cr Mo(ppm) Developed A 0.180 0.51 0.72 0.008 0.002 0.09 0.51 2.63 0.35 104Steel B 0.180 1.00 0.40 0.009 0.001 0.07 0.16 2.97 0.33 137 C 0.182 1.440.33 0.008 0.001 0.08 0.11 2.99 0.31 101 Comparative D 0.188 0.26 0.420.008 0.002 0.11 3.87 1.96 0.65 123 Steel E 0.180 0.24 0.76 0.005 0.0110.30 1.03 2.22 0.46 67 F 0.097 1.45 0.41 0.002 0.005 0.11 0.16 2.95 0.2979 G 0.120 2.00 0.36 0.002 0.005 0.11 0.17 3.02 0.31 69 H 0.199 1.970.41 0.010 0.005 0.11 0.17 4.42 0.31 166 I 0.241 1.53 0.40 0.010 0.0050.11 0.16 2.97 0.31 82

With respect to steel types shown in Table 1, A, B, C designate steelplates prepared in accordance with the present invention, while D, E, F,G, H are provided as comparative examples.

The steel type D as one comparative example contains significantly lowerSi and is excessively rich in Ni, thus departing from the preferredrange of the present invention.

The steel type E contains significantly lower Si, thus also departingfrom the range defined according to the present invention.

The steel types F and G contains lower C, as such departing from thepreferred range of the present invention, and the steel type I exhibitsan unduely high content of C, thus also departing from the desired rangeof the present invention. The steel type H is excessively rich in Cr, assuch departing from the preferred range of the present invention.

TABLE 2 Examples Hot rolling conditions Tensile test Steel Nos. SteelType εc FDT CT TS EI TS * EI S/W properties Delayed fracture Note 1 A0.85 975 655 778 24.4 18,983 ◯ ◯ Comparative example 2 A 0.80 1,006 6301,239 13.6 16,850 ◯ ◯ Developed steel 3 B 0.99 1,024 595 1,274 14.818,855 ◯ ◯ Developed steel 4 B 0.45 925 610 1,360 13.1 17,816 ◯ XComparative example 5 B 0.45 960 600 1,335 13.0 17,355 ◯ X Comparativeexample 6 C 0.84 1,000 610 1,319 14.9 19,653 ◯ ◯ Developed steel 7 D0.92 900 410 1,440 12.0 17,280 X X Comparative example 8 E 0.67 907 6361,143 13.0 14,859 ◯ ◯ Comparative example 9 F 0.67 931 637 1,134 13.715,536 ◯ ◯ Comparative example 10 G 0.67 935 638 1,162 13.8 16,036 ◯ ◯Comparative example 11 H 0.67 942 635 1,559 14.6 22,761 X X Comparativeexample 12 I 0.67 943 635 1,357 17.8 24,155 X X Comparative example

Nos. 1 to 6 in Table 2 are examples in which the steel types A, B, C inTable 1, respectively satisfying the preferred range of the presentinvention, are subjected to rolling under various conditions.

No. 1 was prepared by using the steel type A containing 0.51% Si and byemploying the hot rolling coiling temperature of 655° C. In this case,the data of TS (tensile strength)×EL (elongation) is quite preferable,but the tensile strength is 778 MPa, which is undesirably low.

No. 2 was prepared by using the steel type A and employing the hotrolling coiling temperature of 630° C. This example shows the tensilestrength of 1200 MPa and the elongation of 13%, thus exhibitingexcellent properties.

No. 3 was prepared by using the steel type B containing 1.00% Si and byemploying the hot rolling coiling temperature of 595° C. As shown inTable 2, this example is excellent in both of the strength and theelongation. Additionally, this example shows more enhanced properties inboth of the strength and the elongation, as compared with the No. 2example.

No. 4 and No. 5 were prepared under unsatisfied reduction-ratioconditions during the hot rolling, as such these examples exhibitnegative delayed fracture while satisfying the strength and theelongation.

No. 6 was prepared by using the steel type C containing 1.44% Si and byemploying the hot rolling coiling temperature of 610° C. This exampleexhibits excellent properties in both of the strength and theelongation, and is superior to the No. 3 example in both of the strengthand the elongation.

Nos. 7 to 12 were respectively prepared by carrying out hot rolling,using steel types of comparative examples departing from the desiredrange of the composition used in the present invention.

No. 7 was prepared by rolling, using the steel type D containing lowerSi and higher Ni. This comparative example is insufficient in the spotwelding properties (S/W properties) as well as in the delayed fractureproperties.

No. 8 was prepared by using the steel type E containing lower Si, thusexhibiting insufficient strength and poor balance of strength/ductility.

No. 9 and No. 10 were prepared by using the steel types F and G bothcontaining lower C, respectively, as such exhibiting unduely lowerstrength and poor balance of strength/ductility.

No. 11 and No. 12 were prepared by using the steel types H and I bothcontaining excessively high C, thus exhibiting properly higher strengthand good balance of strength/ductility. However, these comparativeexamples are insufficient in the spot welding properties as well as indelayed fracture properties.

The volume fraction of the ferrite particles was measured by observationusing an optical microscope, after polishing a section cut along therolling direction of each steel plate and then subjecting the polishedsurface to nital corrosion. The measurement also used a commerciallyavailable image analyzer.

The volume fraction of the martensite was obtained by measuring themartensite phase expressed by a white color in an image analysis processduring observation using an optical microscope for a position directedto ¼ of the plate thickness direction, after polishing a section cutalong the rolling direction of each steel plate and then etching thepolished surface by using a liquid formed by mixing 1:1 of 4% picricacid-alcohol and 2% sodium pyrophosphate.

The measurement of the retained austenite was carried out by employingthe X-ray diffraction by using Kα ray of Cu. The volume fraction wasdetermined as an average of the volume fraction of the retainedaustenite to be calculated from a combination of data obtained byrespectively measuring integrated intensities of (200), (220) and (311)faces of the austenite phase and those of (200), (211) faces of theferrite phase, after electrolytic polishing for a position directed to½t of the plate thickness direction.

The tensile properties (tensile strength (TS) and elongation (EL)) weremeasured by subjecting each sample to a tensile test, the sample beingformed into the shape in accordance with the JIS No. 5 test piece.

The delayed fracture properties was assessed by observation of eachsample dipped in a 1N hydrochloric acid solution for a predeterminedperiod of time, after forming φ10 mm punch holes with a 12.2% clearancein a central portion subjected to the tensile test, onto which 8% ormore of strain had been loaded.

As described above, the high-strength steel plates obtained by theexamples, which exhibit high strength and high ductility properties in alower alloy composition are suitable for use as components formanufacturing car structures.

For example, the high-strength steel plates according to the presentinvention can be used as quite preferred materials, such as centerpillars for cars, which require highly excellent properties, includingsufficient tensile strength for supporting doors and preventingdeformation upon collision or the like, bendability for press molding,deep drawability, hole extending workability for forming an attachmenthole to be used for associated equipment, and weldability for weldingthe material to another car component.

Although the invention has been described in its preferred embodimentswith a certain degree of particularity, obviously many changes andvariations are possible therein. It is therefore to be understood thatthe present invention may be practiced otherwise than as specificallydescribed herein without departing from the scope and spirit thereof.

1. A high-strength hot rolled steel plate comprising: a retainedaustenite phase in a volume fraction of 5% to 20%; a martensite phase ina volume fraction of 0% to 10%; and a bainite phase in a remainingvolume fraction, wherein a particle size of retained austenite particlesis 1 μm or less, and the retained austenite particles are dispersed in adensity of seven or more particles per 10 μm².
 2. The high-strength hotrolled steel plate according to claim 1, wherein a particle size ofprior austenite particles is 10 μm or less, and an average aspect ratioof the prior austenite particles is 2.0 or less.
 3. The high-strengthhot rolled steel plate according to claim 1, wherein the steel plate hasa composition comprising: C (0.13 to 0.21 (% by weight)), Si (0.5 to2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and remainingcomponents including Fe and other inevitable impurities.
 4. Thehigh-strength hot rolled steel plate according to claim 1, the steelplate has a plate thickness of 1.0 to 3.0 mm and a tensile strength of1200 MPa or greater.
 5. A method for manufacturing a high-strength hotrolled steel plate, comprising the steps of: preparing a steel materialhaving a composition containing: C (0.13 to 0.21 (% by weight)), Si (0.5to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and remainingcomponents including Fe and other inevitable impurities; roughly rollingthe steel material under conditions of: 1250° C. or higher of anextraction temperature of a reheating furnace; 1030° C. or higher of adischarging-side temperature of roughly rolling mills; and 30% or higherof a reduction ratio for each of roughly rolling final three passes;finish rolling of the steel material under conditions of: 950° C. orhigher of a discharging-side temperature of finish rolling mills; 40% orhigher of a reduction ratio for each mill on a finish front-stage, and0.5 or greater of accumulated strain due to a reduction by three rollingmills on a finish rear-stage; and cooling the steel material byair-cooling for 2 to 6 seconds, followed by water-cooling, and coilingthe steel material at a coiling temperature of 550 to 650° C.
 6. Themethod for manufacturing a high-strength hot rolled steel plateaccording to claim 5, wherein upon the finish rolling, a reductionamount of a topmost portion of the steel material is set greater than anexpected reduction amount, in one or more rolling mills other than amill on a final stage, wherein the reduction amount of the top portionis set at a value increased by less than 10% of the expected reductionamount of the rolling mill, and wherein a length to be rolled in anincreased reduction amount is within 5 m as measured from a biting siteof the topmost portion of the steel material, and thereafter thereduction amount is retuned to the expected reduction amount.
 7. Themethod for manufacturing a high-strength hot rolled steel plateaccording to claim 5, wherein a high-grip roll having micro-carbideparticles dispersed on a surface of the high-grip roll is used as aworking roll for each finish rear-stage rolling mill including a finalrolling mill.